I think I've come up with a good way to put in the 'hard to hit' trait on skeletons: if an attack roll is 3 or lower, the skeleton gains 1 level of deflection against the attack. It'd be like having a shield without the need of an extra roll. It could give deflection against multiple attacks, but there's also the chance that it won't deflect at all. They keep a hand free this way, but can't shove like you do with a shield, or get passive cover.

It could get out of hand if I gave them shields on top of this, but I don't have (or plan to get) any for them.

Hand weapons or ranged weapons when standing still and aiming can still hit with a 2. Heavy weapons would miss on a 2, but you'd still get the deflection if they roll a 3 (the minimum to hit). They'd never get deflection against two-handed weapons, but this is fine to me, as two-handed weapons already only hit half of the time in the hands of regular minifigs, you want them to pack a punch when they hit.

@ Zuppon: yeah, I might add a 'if hit by bladed weapons' or something along those lines disclaimer.